1. Field of the Invention
This invention relates to self-erecting, coiled metal tubes, and more specifically concerns self-erecting metal tubes that include a helically wound ribbon of spring material having adjacent turns stressed to coil in tight overlapping and telescoping engagement and means for controlling the extension of such tubes.
2. Description of the Prior Art
Conventional self-erecting, coiled metal tubes comprise a helically wound ribbon of spring material, with the ribbon having adjacent turns stressed to coil in tight overlapping and telescoping engagement. Examples of such self-erecting metal tubes are disclosed in U.S. Pat. No. 3,587,658, issued June 28, 1971 to Giltner, for an invention in a SELF-ERECTING SPIRAL TUBE DEVICE, and in U.S. Pat. No. 3,860,041, issued Jan. 14, 1975 to Leiter for an invention in a SELF-ERECTING TUBE, which are both owned by the Assignee of the present invention, and which are both incorporated herein by reference.
Self-erecting, coiled metal tubes have the advantage over seamless tubes of exhibiting substantially greater stiffness and a markedly higher bending failure load capability for a given thickness of material and a given outside dimension.
Further, a self-erecting, coiled metal tube gives superior performance in axial loading, both in tension or compression, in that it is "fail-safe" since a load greatly in excess of its normal design load does not damage it. A tension load elongates the tube, and a compressive axial load compresses the tube. Removal of the excess load permits the coiled tube to restore itself to normal length and normal load-bearing performance.
Further, the coiled tube is advantageous in that it may be compressed in a very small space for storage. When a spring metal is employed, the coiled construction of the tube permits the employment of material having a much higher yield strength than is available in materials used to form seamless tubes; for example, a yield of 260 thousand p.s.i. as against 160 thousand p.s.i.
The self-erecting metal tube is stored in a coiled or compressed state, and, when in the coiled state, contains a large amount of stored energy. The compressed tube is normally attached at its bottom to a base, and is extended by releasing it from its coiled state so that its stored energy forces the tube to expand or extend. The tube expands or extends at a constant rate and reaches its full extension in a very short period of time, usually milliseconds. When the tube is in its extended position, adjacent turns of ribbon overlap to give the extended tube its strength.
A problem with conventional self-erecting metal tubes is that they may overextend when they are expanded so that there is little or no overlap between adjacent turns of the ribbon.
It is conventional to control the length of extension of conventional self-erecting metal tubes by a restraint line, or tether, made of braided nylon or dacron with a diameter of one-thirtysecond to one-eighth of an inch. The tether may also be made from Kevlar man-made fibers or stainless steel braided aircraft cables. Kevlar is a registered trademark of E. I. duPont de Nemours and Company of Wilmington, Del.
The tether may be 20 to 30 feet long, and one end may be attached to the innermost turn of the ribbon which forms the top end of the tube when it is erected, and the other end of the tether may be attached to a base support so that, when the tube is expanded, the extension of the ribbon is limited by the length of the tether which is located inside the tube.
Using a tether presents several problems. One problem is storing the tether. The tether is usually stored by wrapping it around a spool or bobbin which may be mounted on the housing at the bottom of the tube, which adds to the overall size of the self-erecting metal tube package. Self-erecting metal tubes are often used where there is very limited space for storing or using a tether.
Another problem is that when the self-erecting metal tube is repackaged and compressed from its extended position to its original coiled compressed state, the tether must be rewound on the bobbin, which adds to the time it takes to repackage the tube.
Still another problem is that the tether and bobbin add to the manufacturing cost.
It has long been desired to control the extended length of a self-erecting metal tube without using a tether.